"Refractory materials" include refractory carbides, borides and nitrides. Refractory materials are characterized by relatively high melting temperatures and hardness, and relatively low chemical reactivity in comparison with non-refractory materials.
Refractory carbides include those transition metal carbides known as interstitial carbides, and the covalent carbides. Interstitial carbides and covalent carbides are collectively referred to herein as "refractory carbides" to distinguish them from carbides formed by the metals of Groups I, II and III of the periodic chart of elements, which form salt-like carbides. Refractory carbides are also to be distinguished from those carbides formed by transition metals having atomic radii smaller than about 1.3 angstroms, such as chromium, magnesium, iron, cobalt and nickel, which do not form typical interstitial carbides.
Interstitial carbides are carbides formed by transition metals having atomic radii of about 1.3 angstroms or greater. In interstitial carbides carbon atoms occupy the interstices between the metal atoms. The characteristic properties of the metal are not fundamentally altered by interstitial carbide formation. Yet the metallic lattice is stabilized, thus increasing hardness and raising the melting point of the composite containing the carbide in comparison with the metal. Transition metals which form refractory carbides include niobium, tantalum, titanium, zirconium, hafnium, molybdenum, vanadium and tungsten.
The covalent carbides are silicon carbide and boron carbide.
Refractory borides include those transition metal borides known as interstitial borides. Interstitial borides include, for example, titanium boride, tantalum diboride, zirconium diboride, and hafnium boride. Refractory nitrides include interstitial nitrides formed by transition metals such as zirconium nitride, titanium nitride, tantalum nitride, and hafnium nitride, as well as refractory covalent nitrides such as boron nitride and silicon nitride. Transition metals form interstitial boride and nitrides which are analogous to the interstitial transition metal carbides in their extreme hardness, chemical inertness and high melting temperature.
Fine particles of refractory carbides are useful in strengthening matrix materials. For example, a titanium carbide particulate has been used to improve the mechanical properties of aluminum. G. W. Halldin et al., Progress in Powder Metallurgy, 38 (1983) 593-611. The wear resistance of sintered aluminum alloy is improved by the addition of two weight percent of titanium carbide. Wear resistance increases with increasing carbide content up to about eight weight percent titanium carbide.
Other composite materials formed from relatively hard particles dispersed in a relatively soft matrix are known in the art. For example, U.S. Pat. No. 4,402,744 discloses composite materials of carbon particles in an aluminum matrix. In addition to the aluminum matrix and the particulate or fibrous carbon, a third component is included in forming the composite. The third component is a powder of an intermetallic compound of aluminum and tantalum, aluminum and titanium, or aluminum and hafnium. For example, the intermetallic compound may be tantalum aluminide, titanium aluminide, or the like. The ratios of aluminum to carbon, and aluminum to tantalum or titanium, are chosen so that heating the mixture under pressure ("sintering") will yield an aluminum alloy composite having an aluminum matrix in which carbon particles and a refractory carbide selected from titanium, tantalum or hafnium carbide, are dispersed (column 9, line 49-column 11, line 10 and Examples XI, XIV and XX).
The composite materials of this patent have good frictional and strength properties and are useful for applications such as rotary seals in automotive applications, aerospace components and the like. The refractory carbides formed by titanium, tantalum and hafnium are believed to help bond the aluminum matrix to the carbon particles dispersed in the matrix. The process for producing the composite materials disclosed in U.S. Pat. No. 4,402,744 requires that carbon particles, aluminum powder, and a powdered intermetallic compound of aluminum and the refractory carbide forming metal, be molded at elevated temperature and pressure to give the composite material.
The sintering of powdered refractory carbides in the presence of a liquid phase nonferrous matrix or binder is used to prepare abrasives commercially, and has been closely studied. For example, the densification processes which occur during sintering of composite materials formed from nickel and titanium carbide, cobalt and titanium carbide, and cobalt and tungsten carbide, have been investigated. V. N. Eremenko et al. Liquid Phase Sintering (Consultants Bureau, New York 1970) 37-46. In abrasive composites, the carbides often form a substantial proportion by weight of the composite.
Fine powders of refractory carbides may be prepared by gas phase reaction of a metal chloride, such as titanium tetrachloride, with methane in a hydrogen plasma. S. F. Exell et al., "Preparation of Ultra-Fine Powders of Refractory Carbides in an Arc-Plasma," Fine Particles (Second Int.'1 Conf., The Electrochemical Soc., Inc., Princeton, N.J. 1974) 165-177.